An organic light-emitting diode (OLED) is a light-emitting diode employing an organic electroluminescent layer as an active layer. OLED display devices have high luminescent efficiency and long operating lifespans. In comparison with liquid-crystal displays, due to the characteristic of spontaneous emission, a device employing an organic light-emitting diode is free of a back-light source.
Generally, an organic light-emitting device is composed of a light-emission layer sandwiched between a pair of electrodes. When an electric field is applied to the electrodes, the cathode injects electrons into the light-emission layer and the anode injects holes into the light-emission layer. When the electrons recombine with the holes in the light-emission layer, excitons are formed. Recombination of the electron and hole results in light emission.
Depending on the spin states of the hole and electron, the exciton, which results from the recombination of the hole and electron, can have either a triplet or singlet spin state. Luminescence from a singlet exciton results in fluorescence whereas luminescence from a triplet exciton results in phosphorescence. The emissive efficiency of phosphorescence is three times that of fluorescence. Therefore, it is crucial to develop highly efficient phosphorescent material, in order to increase the emissive efficiency of an OLED.